1. Field of the Invention
The present invention relates to a semiconductor memory device and semiconductor memory system, and more particularly to a technology for writing data in a nonvolatile semiconductor memory.
2. Description of the Related Art
In recent years, a resistance variable memory has received attention as an alternative nonvolatile memory device to the flash memory. Examples of the resistance variable memory include a ReRAM (Resistance RAM) that uses a transition metal oxide in a recording layer to store the resistance state thereof nonvolatilely, and a PCRAM (Phase Change RAM) that uses a chalcogenide or the like in a recording layer to utilize the resistance information on the crystalline state (conductor) and the amorphous state (insulator).
Such the nonvolatile semiconductor memory device comprises an array of memory cells each including a variable resistive element and a rectifier such as a diode connected in series at each of cross-points of bit lines and word lines. Accordingly, it is suitable for realizing a high-density memory cell array. Multilayered memory cell arrays may be stacked to increase the memory capacity without increasing the area of the cell array portion (for example, JP 2002-541613T).
The variable resistive element used in the nonvolatile semiconductor memory device of such the type varies the resistance thereof through the state change inside the element such as the migration of metal transition elements or the phase change. Therefore, an increase in times of the state change deteriorates the characteristics, a speed of change from one state to the other is extremely lower than that of change in the opposite direction, and either state is more unstable than the other. Thus, there are various performance deteriorating factors.